Factors determining relations between stand age and catchment water balance in mountain ash forests

Abstract

There is a well-documented empirical relationship between stand age and water yield for mountain ash ( Eucalyptus regnans) forested catchments in the Central Highlands of Victoria, Australia. Catchments covered with old-growth stands of mountain ash yield almost twice the amount of water annually as those covered with re-growth stands aged 25 years. In this paper, we provide a mechanistic hydro-ecologic explanation for this phenomenon. We measured leaf area index (LAI), sapwood area index (SAI) and various water balance components in several mountain ash stands, ranging in age between 5 and 240 years. Sap flow measurements show that sap velocity does not vary appreciably amongst stands of different ages, but a systematic decline in SAI with age produces a concomitant decrease in stand transpiration. The decline in overstory SAI is accompanied by a decline in overstory LAI. Understory LAI increases as the overstory LAI decreases, but this layer transpires at only about 63% of the mountain ash rate on a per unit leaf area basis. Hence, while total stand LAI decreases are modest over time, the trend is for a significant decline in total stand transpiration as the forest ages. Rainfall interception also declined over time and there was some indication that interception per unit leaf area also declined. Such reductions can be explained by lesser turbulent mixing and elevated humidity around the bulk of the leaf area in the mature forest. There were small decreases in forest floor evaporation through time, though this only accounted for about 5–8% of the site water balance. Our water balance measurements agree qualitatively with empirical water yield relationships developed for mountain ash forests, though the magnitudes of change differ.